Research findings on Insensibility (Updated November 2011)

Due to increased USDA enforcement of the Humane Slaughter Act, thousands of animals have been observed and data collected by USDA inspectors and plant personnel. A tiny percentage of animals still have a weak corneal reflex with no other indicators of return to sensibility present. These animals MUST be immediately re-stunnned before scalding or any invasive dressing procedure such as leg removal or skinning is performed. Scientific research shows that an animal with a weak corneal reflex in response to touching with a pen is insensible if no other indicators of return to sensibility are present (Anil 1991, Knudsen 2005, and Rumpl et al 1982). Vogel et al (2010) found that in pigs stunned properly by a cardiac arrest method, in some pigs a weak corneal reflex was present. Other signs of return to sensibility such as spontaneous natural blinking, righting reflex, vocalization, rythmic breathing, and visual tracking were all absent. A corneal reflex must NEVER be confused with natural spontaneous blinking that can be observed in live animals in the yards. Animals with spontaneous natural blinking are sensible. To insure that animals remain completely insensible after stunning and bleeding, good bleeding is essential (Grandin 2001). Increased sampling and measurement is probably detecting faint corneal reflexes that were not detected previously. My previous statement that weak corneal reflexes can be completely abolished in CO2 and electrically stunned animals may be wrong. Since corneal reflexes are the beginning of the process of return to sensibility, effort must be made to reduce them to a very low level and insure that all other signs of return to sensibility are absent. All animals showing a corneal reflex MUST be immediately re-stunned. There is a need to collect data on thousands of animals to determine the incidence of weak corneal reflexes when no other signs of return to sensibility are present in animals stunned with CO2 or electricity. In animals stunned with captive bolt corneal reflexes can be abolished.

There is also a possibility that some stunning failures in one of several thousand animals may be due to biological variability or abnormal nervous system development. In one strange case, 1 of 8000 electrically stunned pigs had a stiff arched back righting reflex 45 to 60 seconds after bleeding and there was no corneal reflex, no blinking in response to light, and no response to a nose prick. Most of these pigs were large, 275 lb (124 kg), and extremely heavy muscled. Another possibility is that their physiology was so overloaded by genetic selection or ractopamine (Paylean) that they went into rigor. The plant employees immediately re-stunned the pigs. The righting reflex was not due to poor bleeding because the effectiveness of bleeding was verified with dissection of the throat area. To insure that animals do not return to sensibility while hanging on the line, a plant employee MUST re-stun all animals that have a corneal reflex or any of the signs of return to sensibility listed in this paper. On a 100 animal audit, ALL animals must have no signs of return to sensibility.

Common mistakes and how to avoid them

  1. Mixing up gasping like a fish out of water with rythmic breathing. Gasping may be present after proper electrical or CO2 stunning. Gasping must be absent after captive bolt. Rythmic breathing is a sign of return to sensibility after ALL methods of stunning.
  2. Mixing up nystagmus (vibrating eye) with natural blinking. Natural blinking is a sign of return to sensibility after ALL methods of stunning. Look at live animals in the yards to help you differentiate between nystagmus and natural blinking.
  3. Mixing up a momentary flop of the head of a properly stunned animal with a true righting reflex. A true righting reflex is a sign of return to sensibility after ALL methods of stunning. When a righting reflex occurs, the neck will be stiff. During a momentary flop, the head will be floppy like a wet rag.
  4. When the tongue is extended, soft, and flacid, the animal will be properly stunned and insensible after all methods of stunning. However some properly stunned animals may have the tongue stuck inside the mouth.
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